1. Field of the Invention
The present invention relates to a thermally developable light sensitive material, more particularly, it is concerned with a thermally developable light sensitive material which has improved characteristics by the addition of a trivalent and/or a tetravalent cerium compound thereto.
2. Description of the Prior Art
Thermally developable light sensitive materials utilizing a composition containing as essential components an organic silver salt, a small amount of silver halide and a reducing agent are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075. In these light sensitive systems, silver halides remain in the light sensitive material after development and change in color upon light-exposure, i.e., they are not stabilized to light. Nevertheless, these systems produce satisfactory results, as well as the other system wherein residual silver halide receives a stabilizing treatment to light. This is because only a small amount of silver halide is used and a large portion of the silver component is present in the form of white or pale yellow organic silver salts which are stable to light so that they hardly blacken upon light-exposure. Thus, even if coloration results from the decomposition of residual silver halide caused by light-exposure, such slight coloration can hardly be perceived by the eye. In the above-described light sensitive systems, images are formed by the following mechanism: the oxidizing agent (organic silver salts) and the reducing agent incorporated in the light sensitive layer undergo a redox reaction in the presence of a catalytic amount of exposed silver halide when the system is heated to 80.degree. C., preferably up to 100.degree. C., after the completion of image-wise exposure, although the system is inert at ordinary temperature, resulting in the liberation of silver which causes a quick blackening in exposed areas of the light sensitive layer to produce contrast to unexposed areas (background).
In addition, thermally developable light sensitive materials of this kind include those which contain as a photocatalyst a light sensitive complex prepared from silver and a dye instead of the aforesaid silver halide, as disclosed in Japanese Applications (Laid-Open) 4728/71 and 28221/73, and Japanese Patent Publication 25498/74; and those which contain as organic silver salts a high sensitivity organic silver salt and a low sensitivity organic silver salt in combination, and, therefore, can be free of silver halide, as disclosed in Japanese Applications (Laid-Open) 8522/75. Accordingly, these thermally developable light sensitive materials are within those to which the technique of the present invention is applicable.
The addition of mercury compounds to thermally developable light sensitive materials improves the green shelf life thereof as disclosed in U.S. Pat. No. 3,589,903. However, mercury compounds are undesirable because they are, in general, highly toxic. Therefore, it is an important object to find compound of low toxicity which can serve as a substitute for a mercury compound.